The goal of this project is to increase access to normal donor hematopoietic stem cell transplants for patients who lack an HLA- identical sibling donor, and to improve the safety and efficacy of these transplants as therapy for patients with hematological malignancy. Transplants from alternative donors with variable and sometimes undetected HLA mismatching have a higher risk of morbidity and transplant-related mortality (TRM) due to more severe graft- versus-host-disease (GVHD) than HLA identical sibling transplants. Studies outlined in the four specific aims of this project are directed to improving methods and criteria for donor matching, reducing GVHD and TRM, and understanding the nature of the prolonged immune deficiency that frequently occurs in HLA mismatched and unrelated donor transplants. Advances in DNA-based typing technology has allowed us to identify previously unrecognized HLA genetic variations that occur between patients and their haploidentical or unrelated donor. In the studies proposed here we will determine the degree of mismatching for HLA-A,B,C,DQ and DP alleles that can be safely tolerated without significantly increasing the risk of severe GVHD, and we will better define the situations where HLA mismatching and the potential for a stronger graft-versus-leukemia (GVL) effect may be beneficial. A clinical trail will be undertaken to determine if TRM might be reduced by improving the rate and quality of engraftment using growth factor-mobilized peripheral blood stem cells (PBSC). New methods for selected T-cell depletion (TCD) of HLA mismatched marrow or PBSC grafts from haploidentical related and unrelated donors will be examined to determine if this approach to GVHD prevention will allow us to safely undertake these kind of transplants for patients unable to find an optimal HLA match. Laboratory studies are proposed to examine the kinetics and diversity of posttransplant T- cell reconstitution especially in patients receiving TCD stem cell grafts by analyzing the expression of T-cell receptor (TCR) VB gene families using reverse-transcriptase (RT) and the polymerase chain reaction (PCR) to amplify individual TCR transcripts. The size variation of the CDR3 region of the TCR encoded by recombinant VDJ segments will be analyzed by spectratyping to identify individual TCR clonotypes as a means for estimating clonal diversity and identifying host-reactive T- cell clones that may be involved in GVHD. This method may provide an objective means for monitoring the effectiveness of immunosuppression therapy in the prevention and treatment of GVHD, and may help determine when tolerance has occurred and patients can be safely withdrawn from immunosuppression therapy.